1. Field of the Invention
This invention concerns a method and apparatus for the geophysical exploration of the conductor layers passing through a drill hole, or adjacent to a drill hole, by means of very low frequency waves and, more particularly, to a method using the variations of an alternating magnetic field component along the drill hole.
2. Description of the Prior Art
There are known methods of geophysical investigations employing very low frequency waves transmitted by an emitter generally near the place where the measurements of the picked-up field parameters are effected. These parameters include the amplitude, the phase or the components in phase and in quadrature with the components of the alternating magnetic or electric field used to detect the conductor zones.
All these methods require special transmitters whose frequencies are varied, or receivers which can be readily transported.
Another known technique uses the waves transmitted by a distant L.F.T. station, since these waves are propagated over long distances and penetrate the ground to a great depth. This implements airborne prospecting by comparing the variations of an L.F.T. electro-magnetic field parameter along low altitude flight paths.
Similarly, it is known to draw up horizontal profiles of the apparent resistivities of the ground by using light-weight, easily handled L.F.T. receivers which are moved over the ground.
However, since these processes do not make it possible to accurately determine the depth of the conductor layers, it has been necessary to have available an L.F.T. transmitter adjacent to the prospected place and to associate therewith a detector of a given parameter which is lowered into a drill hole in order to locate these layers with greater exactness.
To sum up, either one could use known L.F.T. stations on the surface, but just detecting an apparent resistivity giving only brief indications of the conductor zones, or one could accurately determine the depth of these conductor layers using special detectors close to a drill hole, the latter serving as a passage for the measuring apparatus.